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Content filtering / Spam official documentation

Hello,
I have many messages blocket into /opt/zimbra/data/amavisd/quarantine as quarantined because of "bad header" but I can see these messages succesfully delivered and present into inbox. So my questions are:

How is supposed to work the content-filtering feature in zimbra?

Is there an official documentation for content filtering (clamav, spamassassin)? Even in the official wiki I'm not able to find anything.

Is there a way to mark only messages (for example with ***SPAM*** and ***VIRUS***) but NOT quarantine it?